Electrode for electrolysis, and method for producing electrode for electrolysis

ABSTRACT

An electrically conductive substrate contains at least titanium. An intermediate layer is provided on a primary surface of the electrically conductive substrate. A composite layer is provided on the intermediate layer. The composite layer includes tantalum layers and catalyst layers. Each of the catalyst layers contains platinum and iridium. Each of the tantalum layers is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. The tantalum layers and the catalyst layers are alternately stacked one layer by one layer in a thickness direction of the electrically conductive substrate. A bottom layer of the composite layer closest to the primary surface of the electrically conductive substrate is constituted by one tantalum layer of the tantalum layers. A top layer of the composite layer furthest from the electrically conductive substrate is constituted by one catalyst layer of the catalyst layers.

TECHNICAL FIELD

The present disclosure relates generally to electrodes for electrolysis and methods for producing electrodes for electrolysis, and more particularly to an electrode for electrolysis and a method for producing an electrode for electrolysis adapted for use in electrolyzing salt water to generate chlorine.

BACKGROUND ART

There has been known a technique for producing hypochlorous acid that includes electrolyzing diluted saline water, which is obtained by adding salt to tap water, to generate chlorine and reacting thus generated chlorine with water to produce the hypochlorous acid (see Patent Literature 1).

Patent Literature 1 discloses an electrode for electrolysis that includes: an electrode body made from titanium or titanium alloy; a titanium oxide layer provided on the electrode body; an intermediate oxide layer provided on the titanium oxide layer, the intermediate oxide layer being made of a composite that contains iridium oxide within a range of 3 to 30 mol % and tantalum oxide within a range of 70 to 97 mol % in metal conversion; and a composite body provided on the intermediate oxide layer, the composite body containing rhodium oxide within a range of 2 to 35 mol %, iridium oxide within a range of 30 to 80 mol %, tantalum oxide within a range of 6 to 35 mol %, and platinum within a range of 12 to 62 mol % in metal conversion.

The electrode for electrolysis is desired to be less likely to cause the separation of the composite layer, which contains Iridium serving as a catalyst, in order to increase the life-time of the electrode.

CITATION LIST Patent Literature

Patent Literature 1: JP 2009-52069 A

SUMMARY OF INVENTION

An object of the present disclosure is to provide an electrode for electrolysis and a method for producing an electrode for electrolysis, which are less likely to cause the separation of the composite layer.

An electrode for electrolysis according to one aspect of the present disclosure includes an electrically conductive substrate, an intermediate layer, and a composite layer. The electrically conductive substrate contains at least titanium. The intermediate layer is provided on a primary surface of the electrically conductive substrate. The composite layer is provided on the intermediate layer. The composite layer includes a plurality of tantalum layers and a plurality of catalyst layers. Each of the plurality of tantalum layers is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. Each of the plurality of catalyst layers contains platinum and iridium. In the composite layer, the plurality of tantalum layers and the plurality of catalyst layers are alternately stacked one layer by one layer in a thickness direction of the electrically conductive substrate. The composite layer has a bottom layer closest to the primary surface of the electrically conductive substrate, the bottom layer being constituted by one tantalum layer of the plurality of tantalum layers. The composite layer has a top layer furthest from the electrically conductive substrate, the top layer being constituted by one catalyst layer of the plurality of catalyst layers.

A method for producing an electrode for electrolysis according to another aspect of the present disclosure includes an intermediate layer formation process and a composite layer formation process. The intermediate layer formation process includes forming an intermediate layer on a primary surface of an electrically conductive substrate containing titanium. The composite layer formation process includes forming a composite layer on the intermediate layer. The composite layer has a stacked structure alternating a plurality of tantalum layers and a plurality of catalyst layers one layer by one layer. Each of the plurality of tantalum layers is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. Each of the plurality of catalyst layers contains platinum and iridium. The composite layer formation process includes a first process, a second process, and a third process. The first process includes applying a solution containing tantalum onto the intermediate layer and subsequently firing at a first prescribed temperature to form a tantalum layer, of the plurality of tantalum layers, that constitutes a bottom layer of the stacked structure. The second process includes repeating a first step and a second step to form a stacked body serving as a basis of a remaining part, other than the tantalum layer that constitutes the bottom layer, of the stacked structure. The first step includes applying a solution containing platinum and iridium and subsequently heating and drying at a second prescribed temperature to form a layer serving as a basis of one catalyst layer of the plurality of catalyst layers. The second step includes applying a solution containing tantalum and subsequently heating and drying at a third prescribed temperature to form a layer serving as a basis of one, but other than the tantalum layer that constitutes the bottom layer, of the plurality of tantalum layers. The third process includes firing the stacked body at a fourth prescribed temperature, which is higher than each of the second prescribed temperature and the third prescribed temperature, to form the plurality of catalyst layers and tantalum layers, other than the tantalum layer that constitutes the bottom layer, of the plurality of tantalum layers together with a plurality of cracks recessed from a main surface of the catalyst layer, the main surface being a surface away from the intermediate layer.

An electrode for electrolysis according to yet another aspect of the present disclosure includes an electrically conductive substrate, an intermediate layer, and a composite layer. The electrically conductive substrate contains at least titanium. The intermediate layer is provided on a primary surface of the electrically conductive substrate. The composite layer is provided on the intermediate layer. The composite layer includes a plurality of tantalum layers and a plurality of catalyst layers. Each of the plurality of tantalum layers is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. Each of the plurality of catalyst layers contains platinum and iridium. In the composite layer, the plurality of tantalum layers and the plurality of catalyst layers are alternately stacked one layer by one layer in a thickness direction of the electrically conductive substrate. The composite layer has a bottom layer closest to the primary surface of the electrically conductive substrate, the bottom layer being constituted by one tantalum layer of the plurality of tantalum layers. The composite layer has a top layer furthest from the electrically conductive substrate, the top layer being constituted by another one tantalum layer of the plurality of tantalum layers. The composite layer has a main surface away from the intermediate layer. The electrode for electrolysis has a plurality of recesses recessed from the main surface of the composite layer. Each of the plurality of recesses has a depth such that each of the plurality of recesses is made to go completely through at least one catalyst layer of the plurality of catalyst layers.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view of an electrode for electrolysis according to a first embodiment;

FIG. 2 is a plan view of the electrode for electrolysis;

FIGS. 3A to 3D are sectional views illustrating processes of a method for producing the electrode for electrolysis;

FIG. 4 is a graph illustrating a result of a durability test for the electrode for electrolysis;

FIGS. 5A to 5D are views of an inferred mechanism for illustrating that the electrode for electrolysis has an improved durability;

FIG. 6 is a sectional view of an electrode for electrolysis according to a second embodiment;

FIGS. 7A to 7E are sectional views illustrating processes of a method for producing the electrode for electrolysis; and

FIG. 8 is a sectional view of an electrode for electrolysis according to a variation.

DESCRIPTION OF EMBODIMENTS

The FIGS. 1, 2, 3A to 3D, 5A to 5D, 6, 7A to 7E, and 8 to be referred to in the following description of first and second embodiments and the like are all schematic representations. The ratio of the dimensions including thicknesses, of respective constituent elements illustrated on the drawings does not always reflect their actual dimensional ratio.

First Embodiment (1) Overview

An electrode for electrolysis 1 according to the first embodiment is explained with reference to FIGS. 1 to 3D.

The electrode for electrolysis 1 may be an electrode adapted for use in the electrolysis of salt water to generate chlorine. The salt water may be saline water, for example. The electrode for electrolysis 1 may be used for a system for electrolyzing the salt water. Such a system includes an anode and a cathode between which a DC voltage is to be applied from a power source. In this system, the electrode for electrolysis 1 may be used as the anode. This system can electrolyze the saline water to generate chlorine, and can produce hypochlorous acid by reacting the generated chlorine with water.

(2) Components of Electrode for Electrolysis

The electrode for electrolysis 1 includes an electrically conductive substrate 2, an intermediate layer 3, and a composite layer 4.

Components of the electrode for electrolysis 1 will be explained more in detail.

(2.1) Electrically Conductive Substrate

The electrically conductive substrate 2 includes a primary surface 21 (hereinafter, also referred to as “first primary surface 21”), and a second primary surface 22 opposite to the first primary surface 21. The electrically conductive substrate 2 has a plan shape (shape of the outline of the electrically conductive substrate 2 as seen in a thickness direction D1 of the electrically conductive substrate 2) of a rectangle. The electrically conductive substrate 2 has a thickness within a range of 100 μm to 2 mm for example, and the thickness may be 500 μm as an example. The electrically conductive substrate 2 has a size in the plan view of 25 mm by 60 mm, for example.

The electrically conductive substrate 2 contains at least titanium. The electrically conductive substrate 2 is a titanium substrate, for example. The material of the electrically conductive substrate 2 may be titanium or an alloy containing titanium as main component (hereinafter, referred to as “titanium alloy”). Examples of the titanium alloy includes titanium-palladium alloy, titanium-nickel-ruthenium alloy, titanium-tantalum alloy, titanium-aluminum alloy, titanium-aluminum-vanadium alloy, and the like.

The first primary surface 21 of the electrically conductive substrate 2 may preferably be uneven. This can contribute to increase the adhesion with respect to the intermediate layer 3. According to the electrode for electrolysis 1 of the first embodiment, the first primary surface 21 of the electrically conductive substrate 2 is roughened before the intermediate layer 3 is provided. In terms of the surface roughness of the first primary surface 21 of the electrically conductive substrate 2, the arithmetic mean deviation of the roughness profile Ra thereof is 0.3 μm and the maximum height of the profile Rz is 3 μm, for example. The arithmetic mean deviation of the roughness profile Ra and the maximum height of the profile Rz are defined by the JIS B 0601-2001 (ISO 4287-1997) standard. The arithmetic mean deviation of the roughness profile Ra and the maximum height of the profile Rz can be determined based on the measurement result for the Cross-sectional Scanning Electron Microscope (SEM) Image, for example.

(2.2) Intermediate Layer

The intermediate layer 3 is provided on the first primary surface 21 of the electrically conductive substrate 2. The electrode for electrolysis 1 has a boundary face between the electrically conductive substrate 2 and the intermediate layer 3. The intermediate layer 3 has corrosion resistance to the salt water and the chlorine. The intermediate layer 3 may preferably be made of material exhibiting corrosion resistance to the salt water and the chlorine superior to those of the electrically conductive substrate 2. The intermediate layer 3 may preferably be made of material having electrical conductivity and high electrical conduction property. This can contribute to increase the electrical conductivity of the electrode for electrolysis 1. The material of the intermediate layer 3 may be transition metal or a mixture containing transition metal, for example. Examples of the material include: platinum; a mixture of tantalum, platinum and iridium; iridium; iridium oxide; nickel; and the like. A specific example of the material of the intermediate layer 3 is platinum. The intermediate layer 3 has a thickness within a range of 0.3 μm to 5 μm for example, and the thickness may be 0.6 μm as an example.

(2.3) Composite Layer

The composite layer 4 is provided on the intermediate layer 3. The electrode for electrolysis 1 has a boundary face between the composite layer 4 and the intermediate layer 3. The composite layer 4 is provided on the electrically conductive substrate 2 with the intermediate layer 3 interposed therebetween.

The composite layer 4 includes a plurality of (four, in the illustrated example) tantalum layers 41, and a plurality of (four, in the illustrated example) catalyst layers 42. Each of the plurality of catalyst layers 42 contains platinum and iridium. Each of the plurality of catalyst layers 42 is constituted by a mixture of platinum and iridium. Each of the plurality of tantalum layers 41 is a layer made from tantalum oxide, but is not limited thereto. Alternatively, a tantalum layer 41 may be a layer made from tantalum or a layer made from a mixture of tantalum oxide and tantalum (i.e., a layer including a mix of tantalum oxide and tantalum). In each of the plurality of catalyst layers 42, iridium is dispersed through platinum. Iridium serves as a catalyst for the generation reaction of chlorine. The composite layer 4 has a platinum:iridium:tantalum molar ratio of 6-10:1-10:1-8, for example. The molar quantity of iridium may preferably be smaller or equal to the molar quantity of platinum. This can contribute to suppress the cohesion of iridium due to aging variation caused by the long time use of the electrode for electrolysis 1. The tantalum layer 41 is superior to the catalyst layer 42 in the corrosion resistance and is resistant to structural change. Accordingly, a tantalum layer 41 provided on a catalyst layer 42 can prevent the catalyst layer 42 directly underneath from causing the elution of iridium.

The composite layer 4 has a stacked structure alternating the plurality of tantalum layers 41 and the plurality of catalyst layers 42 one layer by one layer in the thickness direction D1 of the electrically conductive substrate 2. Each of the plurality of tantalum layers 41 has a thickness within a range of 15 nm to 300 nm for example, and the thickness may be 100 nm as an example. Each of the plurality of catalyst layers 42 has a thickness within a range of 15 nm to 100 nm for example, and the thickness may be 50 nm as an example.

Hereinafter, for the convenience of the explanation, the four tantalum layers 41 may be referred to as a first tantalum layer 411, a second tantalum layer 412, a third tantalum layer 413, and a fourth tantalum layer 414, respectively, in the order from a side closer to the first primary surface 21 of the electrically conductive substrate 2. The four catalyst layers 42 may be referred to a first catalyst layer 421, a second catalyst layer 422, a third catalyst layer 423, and a fourth catalyst layer 424, respectively, in the order from the side closer to the first primary surface 21 of the electrically conductive substrate 2.

In the composite layer 4, the first tantalum layer 411, the first catalyst layer 421, the second tantalum layer 412, the second catalyst layer 422, the third tantalum layer 413, the third catalyst layer 423, the fourth tantalum layer 414, and the fourth catalyst layer 424 are arranged in this order from the side of the electrically conductive substrate 2.

The composite layer 4 has a bottom layer closest to the primary surface 21 of the electrically conductive substrate 2. The bottom layer of the composite layer 4 is constituted by one tantalum layer 41 of the plurality of tantalum layers 41. The composite layer 4 has a top layer furthest from the electrically conductive substrate 2. The top layer of the composite layer 4 is constituted by one catalyst layer 42 of the plurality of catalyst layers 42.

In the composite layer 4, one tantalum layer 41 (first tantalum layer 411) of the plurality of tantalum layers 41 constitutes the bottom layer that is closest to the first primary surface 21 of the electrically conductive substrate 2. In the composite layer 4, one catalyst layer 42 (fourth catalyst layer 424, in the illustrated example) of the plurality of catalyst layers 42 constitutes the top layer that is furthest from the electrically conductive substrate 2.

The electrode for electrolysis 1 has a plurality of recesses 5 recessed from a main surface 40 that is a surface, away from the intermediate layer 3, of the composite layer 4. Each of the plurality of recesses 5 has a depth which is larger than a distance L1 and also smaller than or equal to a distance L2. The distance L1 is a distance between the main surface 40 of the composite layer 4 and a catalyst layer 42 (third catalyst layer 423), second furthest from the electrically conductive substrate 2, of the plurality of catalyst layers 42. The distance L2 is a distance between the main surface 40 of the composite layer 4 and the intermediate layer 3.

Each of the plurality of recesses 5 has a width H1 (see FIG. 2), in the plan view as seen in the thickness direction D1 of the electrically conductive substrate 2, that falls within a range of 0.1 μm to 10 μm, and the width may preferably fall within a range of 0.3 μm to 3 μm. The width H1 of the recess 5, in the plan view as seen in the thickness direction D1 of the electrically conductive substrate 2, indicates an opening width of the recess 5 in a transverse direction (perpendicular to the longitudinal direction) thereof within the main surface 40 of the composite layer 4.

Moreover, a percentage of S2 with respect to S1+S2 falls within a range of 5% to 50% for example, where S1 denotes an area of the main surface 40 of the composite layer 4 in the plan view as seen in the thickness direction D1 of the electrically conductive substrate 2, and S2 denotes a total area of opening areas of the plurality of recesses 5 in the main surface 40 of the composite layer 4 in the plan view as seen in the thickness direction D1 of the electrically conductive substrate 2. The percentage of S2 with respect to S1+S2 may preferably be 5% or more, which can contribute to improve the chlorine generation efficiency. The percentage of S2 with respect to S1+S2 may preferably be 50% or less, and may further preferably be 20% or less, which can contribute to suppress the separation of the composite layer 4. That is, the percentage of S2 with respect to S1+S2 may preferably fall within a range of 5% to 20%. In the electrode for electrolysis 1, at least one recess of the plurality of recesses is present in a 0.01 mm² square region in the plan view as seen in the thickness direction D1 of the electrically conductive substrate 2. A total length of each opening edge of the at least one recess present in the 0.01 mm² square region is greater than or equal to 1 mm.

(3) Method for Producing Electrode for Electrolysis

An example of a method for producing the electrode for electrolysis 1 is explained with reference to FIGS. 3A to 3D.

The method for producing the electrode for electrolysis includes preparing the electrically conductive substrate 2 as shown in FIG. 3A. The method includes a roughening process, an intermediate layer formation process, and a composite layer formation process which are performed sequentially after the preparation.

The roughening process includes, for example, immersing the electrically conductive substrate 2 in an oxalic acid aqueous solution to roughen the first primary surface 21 of the electrically conductive substrate 2. The roughening process is optional and may be omitted. In terms of the surface roughness of the first primary surface 21 of the electrically conductive substrate 2 after the roughening process, the arithmetic mean deviation of the roughness profile Ra thereof is 0.3 μm and the maximum height of the profile Rz thereof is 3 μm, for example. The arithmetic mean deviation of the roughness profile Ra and the maximum height of the profile Rz may be values measured with a surface roughness meter of Zygo Co.

The intermediate layer formation process includes forming the intermediate layer 3 on the first primary surface 21 of the electrically conductive substrate 2 (see FIG. 3B). The intermediate layer 3 is a platinum layer, for example. The intermediate layer formation process includes: applying a raw material solution, serving as a basis of the intermediate layer 3, onto the first primary surface 21 of the electrically conductive substrate 2; performing natural drying; performing heat treatment; and firing, to form the intermediate layer 3. The raw material solution is a solution obtained by dissolving a platinum compound in a solvent. The solvent is a liquid of a mixture of ethylene glycol monoethyl ether, hydrochloric acid and ethanol, for example. The platinum compound is for example hydrogen chloroplatinate, but is not limited thereto. Alternatively, the platinum compound may be platinum chloride, for example. The formation method of the intermediate layer 3 is not limited to the above method, but may include vapor deposition, sputtering, CVD method, plating, and the like.

The compound layer formation process includes forming the composite layer 4 on the intermediate layer 3 (see FIG. 3D).

The compound layer formation process includes a stacked body formation process and a firing process.

The stacked body formation process includes performing a first prescribed number of times (four times, for example) of first steps and a second prescribed number of times (four times, for example) of second steps, where the first step and the second step are performed alternately, to form, on the intermediate layer 3 which is on the electrically conductive substrate 2, a stacked body 400 (see FIG. 3C) serving as a basis of the composite layer 4.

The first step includes applying a solution (hereinafter, referred to as “first solution”) containing a tantalum compound serving as a basis of the tantalum layer 41, and subsequently performing, without natural drying, heat treatment (drying process) of heating and drying under a first condition, to form a first material layer 410 serving as a basis of one tantalum layer 41 of the plurality of tantalum layers 41. The first solution is a solution obtained by dissolving the tantalum compound in a solvent (hereinafter, referred to as “first solvent”). The first solvent is a liquid of a mixture of ethylene glycol monoethyl ether, hydrochloric acid and ethanol, for example. The tantalum compound is for example tantalum chlorine, but is not limited thereto. Alternatively, the tantalum compound may be tantalum ethoxydo, for example. The metal concentration (tantalum concentration) of the first solution is 26 mg/L, for example. The application amount of the first solution is 1 μL/cm², for example. The first condition includes a heat treatment temperature and a heat treatment time. The heat treatment temperature of the first condition falls within a range of 100° C. to 400° C. for example, and the heat treatment temperature may be 220° C. as an example. The heat treatment time of the first condition falls within a range of 5 minutes to 15 minutes for example, and the heat treatment time may be 10 minutes as an example.

The second step includes applying a solution (hereinafter, referred to as “second solution”) containing a platinum compound and iridium compound serving as a basis of the catalyst layer 42, and subsequently performing, without natural drying, heat treatment (drying process) of heating and drying under a second condition, to form a second material layer 420 serving as a basis of one catalyst layer 42 of the plurality of catalyst layers 42. The second solution is a solution obtained by dissolving the platinum compound and the iridium compound in a solvent (hereinafter, referred to as “second solvent”). The second solvent is a liquid of a mixture of ethylene glycol monoethyl ether, hydrochloric acid and ethanol, for example. The platinum compound is for example hydrogen chloroplatinate, but is not limited thereto. Alternatively, the platinum compound may be platinum chloride, for example. The iridium compound is for example hydrogen chloroidiate, but is not limited thereto. Alternatively, the iridium compound may be iridium chloride, iridium nitrate, or the like, for example. The metal concentration (total of platinum concentration and iridium concentration) of the second solution is 26 mg/L, for example. The application amount of the second solution is 2 μL/cm², for example. The second condition includes a heat treatment temperature and a heat treatment time. The heat treatment temperature of the second condition falls within a range of 100° C. to 400° C. for example, and the heat treatment temperature may be 220° C. as an example. The heat treatment time of the second condition falls within a range of 5 minutes to 15 minutes for example, and the heat treatment time may be 10 minutes as an example.

The firing process includes performing thermal treatment of firing the stacked body 400 under a predetermined firing condition to form the composite layer 4 and a plurality of cracks (recesses 5) (see FIG. 3D). The firing condition includes a firing temperature and a firing time. The firing temperature falls within a range of 500° C. to 700° C. for example, and the firing temperature may be 560° C. as an example. The firing time falls within a range of 10 minutes to 20 minutes for example, and the firing time may be 15 minutes as an example. The plurality of cracks (recesses 5) may have mutually different shapes. The crack may be formed along a thickness direction of the composite layer 4 or may be at least partially curved in the composite layer 4.

(4) Example

FIG. 4 is a graph that shows durability test results for the following nine examples, Examples 1 to 9. The nine examples have mutually different electrodes for electrolysis 1 whose platinum (Pt):iridium (Ir):tantalum (Ta) molar ratio of the composite layers 4 and/or the number of layers (the total number of layers of the tantalum layer 41 and the catalyst layer 42) included in the stacked structures of the composite layers 4 are different from each other, where:

Example 1 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:1:6 and the number of layers is 10;

Example 2 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:1:6 and the number of layers is 20;

Example 3 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:1:6 and the number of layers is 30;

Example 4 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:3:6 and the number of layers is 10;

Example 5 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:3:6 and the number of layers is 20;

Example 6 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:3:6 and the number of layers is 30;

Example 7 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:5:6 and the number of layers is 10;

Example 8 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:5:6 and the number of layers is 20; and

Example 9 relates to a sample whose molar ratio of Pt:Ir:Ta is 8:5:6 and the number of layers is 30.

The durability test was conducted according to the accelerated test. In the durability test for each example, two electrodes for electrolysis 1 were produced in a same condition and used as a pair of electrodes. Firstly, the pair of electrodes were immersed in salt water inside a tank for electrolytic cell for a durability test apparatus, and an electric current was supplied between the pair of electrodes to conduct an initial aging. After the initial aging, an electric current was continuously provided for a certain duration of time between the pair of electrodes immersed in the salt water, subsequently the pair of electrodes were put and immersed in another salt water inside a tank for electrolytic cell for a chlorine concentration measurement, and then an electric current was provided for a predetermined time (3 minutes) between the pair of electrodes and (average) chlorine concentration around the electrode were measured, which were performed repeatedly. It should be noted that the tank for electrolytic cell for the durability test apparatus has an inlet and an outlet for the salt water. During the durability test, salt water was added to the tank such that the electrical conductivity of the salt water inside the tank for electrolytic cell for the durability test apparatus was maintained within a range of 1650±200 S/m. During the durability test, the tank for electrolytic cell for the durability test apparatus was continuously supplied with tap water with a flow rate of 2 L/min while discharging water therefrom. The salt water supplied in the tank for electrolyte cell for the durability test apparatus was obtained by dissolving common salt (sodium chloride) in tap water. The current value of the electric current supplied during the durability test was 400 mA. Moreover, the salt water inside the tank for electrolytic cell for the chlorine concentration measurement was obtained by dissolving 4.5 g of common salt (sodium chloride) in 800 mL of pure water. The current value of the electric current supplied during the chlorine concentration measurement was 400 mA. Furthermore, during the initial aging, the electric current was supplied between the pair of electrodes for total twelve minutes, where the polarity was reversed every time a predetermined time (3 minutes) elapses. The feature “the polarity is reversed” used herein indicates that the roles of the pair of electrodes, the anode or the cathode, are mutually interchanged. In other words, the feature “the polarity is reversed” indicates that an electrode (to be) used as the higher potential side electrode is changed from one of the pair of electrodes to the other thereof, such that one of the electrodes which has been used as the anode is to be used as the cathode, and vice verse.

The horizontal axis of FIG. 4 indicates the durability test time (elapsed time) after the initial aging. The vertical axis of FIG. 4 indicates the chlorine concentration. It should be noted that the chlorine generated around the anode is to be used for the generation of hypochlorous acid. Thus, the chlorine concentration substantially reflects the amount of chlorine, which has been produced during a recent unit time.

It can be understood from FIG. 4 that the durability increases as an increase in the number of layers under the same platinum (Pt):iridium (Ir):tantalum (Ta) molar ratio. It can also be understood from FIG. 4 that the durability increases as an increase in the percentage of iridium (Ir) under the same number of layers and the same platinum (Pt):tantalum (Ta) molar ratio.

Based on FIGS. 5A, 5B, 5C and 5D, an inferred mechanism is described which can explain a reason why the electrode for electrolysis 1 according to the first embodiment has an improved durability. FIGS. 5A, 5B, 5C, and 5D are ordered according to the time series.

According to the electrode for electrolysis 1 in a state shown in FIG. 5A, respective sides of a plurality of (four) catalyst layers 42 that partially form an inner surface of the recess 5, as well as the main surface 40, of the composite layer 4 are in contact with the salt water. Each of the plurality of (four) catalyst layers 42 thus can contribute to the generation of the chlorine.

FIG. 5B shows a state of the electrode for electrolysis 1 where a catalyst layer 42 (fourth catalyst layer 424) of the top layer shown in FIG. 5A is lost. According to the state shown in FIG. 5B, respective sides of a plurality of (three) catalyst layers 42 that partially form an inner surface of the recess 5 are in contact with the salt water. Each of the plurality of (three) catalyst layers 42 thus can contribute to the generation of the chlorine.

FIG. 5C shows a state where the plurality of (three) catalyst layers 42 are partially lost in an in-plane direction, from the state shown in FIG. 5B. The in-plane direction is defined as a direction perpendicular to the thickness direction D1 of the electrically conductive substrate 2. That is, the in-plane direction is a direction along the first primary surface 21 of the electrically conductive substrate 2. According to the state shown in FIG. 5C, respective sides, closer to the recess 5, of the plurality of (three) catalyst layers 42 are in contact with the salt water. Each of the plurality of (three) catalyst layers 42 thus can contribute to the generation of the chlorine.

FIG. 5D shows a state where a tantalum layer 41 (fourth tantalum layer 414) on a catalyst layer 42 (third catalyst layer 423), which is furthest from the electrically conductive substrate 2, of the plurality of (three) catalyst layers 42 is partially lost along the in-plane direction, from the state shown in FIG. 5C. According to the state shown in FIG. 5D, respective sides, closer to the recess 5, of the plurality of (three) catalyst layers 42, as well as a main surface of the third catalyst layer 423 closer to the fourth tantalum layer 414, are in contact with the salt water. Each of the plurality of (three) catalyst layers 42 thus can contribute to the generation of the chlorine.

With the electrode for electrolysis 1 according to the first embodiment, at least one of the catalyst layers 42 always can contribute to the generation of the chlorine, regardless of the change in the state thereof. The electrode for electrolysis 1 according to the first embodiment thus can have an improved durability.

(5) Effect

The electrode for electrolysis 1 according to the first embodiment includes the composite layer 4 alternating the plurality of tantalum layers 41 and the plurality of catalyst layers 42 one layer by one layer, which can contribute to suppress the separation of the composite layer 4. The electrode for electrolysis 1 according to the first embodiment includes the composite layer 4, which also can contribute to suppress the waste of the composite layer 4 in use. The electrode for electrolysis 1 according to the first embodiment includes the composite layer 4, which also can contribute to suppress the cohesion of iridium.

The electrode for electrolysis 1 according to the first embodiment has the plurality of recesses 5, which can increase the area of the surface of the composite layer 4 contributing the generation of the chlorine, and can contribute to improve the chlorine generation efficiency.

Second Embodiment

An electrode for electrolysis 1 a according to the second embodiment is explained with reference to FIG. 6.

The electrode for electrolysis 1 a according to the second embodiment is substantially the same as the electrode for electrolysis 1 according to the first embodiment. The electrode for electrolysis 1 a according to the second embodiment differ from the electrode for electrolysis 1 according to the first embodiment in the depths of the recesses 5. Components of the electrode for electrolysis 1 a according to the second embodiment similar to those of the electrode for electrolysis 1 according to the first embodiment are assigned same reference signs and explanation thereof may be omitted.

Each of the plurality of recesses 5 of the electrode for electrolysis 1 a according to the second embodiment has a depth which is smaller than or equal to a distance L3 between a main surface 40 of a composite layer 4 and a bottom layer (first tantalum layer 411) of the composite layer 4. This can contribute to further suppression of the separation of the composite layer 4 according to the electrode for electrolysis 1 a according to the second embodiment.

A method for producing the electrode for electrolysis 1 a according to the second embodiment is explained with reference to FIGS. 7A to 7E. It may be omitted the detailed explanation of some of processes, similar to those of the method for producing the electrode for electrolysis 1 according to the first embodiment.

An electrically conductive substrate 2 is prepared firstly as shown in FIG. 7A, and a roughening process, an intermediate layer formation process, and a composite layer formation process are performed sequentially after the preparation.

The roughening process includes, for example, immersing the electrically conductive substrate 2 in an oxalic acid aqueous solution to roughen a first primary surface 21 of the electrically conductive substrate 2. The roughening process is optional and may be omitted.

The intermediate layer formation process includes forming an intermediate layer 3 on the first primary surface 21 of the electrically conductive substrate 2 (see FIG. 7B).

The compound layer formation process includes forming a composite layer 4 on the intermediate layer 3 (see FIG. 7E).

The compound layer formation process includes a first process, a second process, and a third process.

The first process includes applying a solution containing tantalum, serving as a basis of a tantalum layer 41, onto the intermediate layer 3, and subsequently performing a firing to form a tantalum layer 41, constituting a bottom layer of a stacked structure of the composite layer 4, of a plurality of tantalum layer 41 (see FIG. 7C). The solution is obtained by dissolving a tantalum compound in a solvent, for example. The solution thus includes tantalum. The solvent is a liquid of a mixture of ethylene glycol monoethyl ether, hydrochloric acid and ethanol, for example. The tantalum compound is for example tantalum chlorine, but is not limited thereto. Alternatively, the tantalum compound may be tantalum ethoxydo, for example. Further alternatively, the solution may be obtained by dissolving pure tantalum in the solvent, for example, which can form the tantalum layer 41 as a layer made from tantalum, instead of a layer made from tantalum oxide. The metal concentration (tantalum concentration) of the solution is 26 mg/L, for example. The application amount of the solution is 1 μL/cm², for example. The firing condition includes a firing temperature (first prescribed temperature) and a firing time. The firing temperature falls within a range of 500° C. to 700° C. for example, and the firing temperature may be 560° C. as an example. The firing time falls within a range of 10 minutes to 20 minutes for example, and the firing time may be 15 minutes as an example.

The second process includes a first prescribed number of times (e.g., four times) of first steps and a second prescribed number of times (e.g., three times) of second steps which are performed alternatively to form a stacked body 401. The stacked body 401 serves as a basis of a remaining part, other than the tantalum layer 41 constituting the bottom layer, of the stacked structure of the composite layer 4 (see FIG. 7D).

The first step of the second process includes applying a second solution containing a platinum compound and iridium compound serving as a basis of a catalyst layer 42, and subsequently performing, without natural drying, heat treatment (drying process) of heating and drying under a second condition to form a second material layer 420 serving as a basis of one catalyst layer 42 of the plurality of catalyst layers 42. The second solution contains platinum and iridium. The second solution is applied to a layer (a tantalum layer 41 as a bottom layer, or a first material layer 410 described later, for example) exposed outside on a side of the first primary surface 21 of the electrically conductive substrate 2. The second condition includes a heat treatment temperature and a heat treatment time. The heat treatment temperature (second prescribed temperature) of the second condition falls within a range of 100° C. to 400° C. for example, and the heat treatment temperature may be 220° C. as an example. The heat treatment time of the second condition falls within a range of 5 minutes to 15 minutes for example, and the heat treatment time may be 10 minutes as an example.

The second step of the second process includes applying a first solution containing a tantalum compound serving as a basis of the tantalum layer 41, and subsequently performing, without natural drying, heat treatment (drying process) of heating and drying under a first condition, to form a first material layer 410 serving as a basis of one tantalum layer 41 of the plurality of tantalum layers 41. The first solution contains tantalum. The first solution is applied to a layer (a second material layer 420) exposed outside on a side of the first primary surface 21 of the electrically conductive substrate 2. The first condition includes a heat treatment temperature and a heat treatment time. The heat treatment temperature (third prescribed temperature) of the first condition falls within a range of 100° C. to 400° C. for example, and the heat treatment temperature may be 220° C. as an example. The heat treatment time of the first condition falls within a range of 5 minutes to 15 minutes for example, and the heat treatment time may be 10 minutes as an example.

The third process includes firing the stacked body 401 at a prescribed temperature (fourth prescribed temperature) to form the plurality of catalyst layers 42 and tantalum layers 41, other than the tantalum layer 41 constituting the bottom layer, of the plurality of the tantalum layer 41 together with a plurality of cracks (recesses 5) recessed from a main surface 40 of the catalyst layer 42. The main surface 40 is a surface away from the intermediate layer 3 (see FIG. 7E).

The method for producing the electrode for electrolysis 1 a according to the second embodiment can provide the electrode for electrolysis 1 a, which is less likely to cause the separation of the composite layer 4.

The first and second embodiments are only exemplary ones of various embodiments of the present disclosure. The exemplary embodiment may be readily modified in various manners depending on a design choice or any other factor, as long as the purpose of the present disclosure can be attained.

In a variation, a plan shape of an electrically conductive substrate 2 is not limited to a rectangle, but may be a square for example.

In a variation, the number of tantalum layers 41 and/or catalyst layers 42 of a composite layer 4 is not limited to four, but may be two, three, five or more. The number of tantalum layers 41 and the number of catalyst layers 42 in the composite layer 4 are not limited to be the same, but may be different from each other.

In a variation, thicknesses of a plurality of tantalum layers 41 are not limited to be the same, but may be different from each other. It is also possible that some of a plurality of tantalum layers 41 have the same thickness, and remaining of the plurality of tantalum layer 41 may have a thickness different therefrom.

In a variation, thicknesses of a plurality of catalyst layers 42 are not limited to be the same, but may be different from each other. It is also possible that some of a plurality of catalyst layers 42 have the same thickness, and remaining of the plurality of catalyst layer 42 may have a thickness different therefrom.

In a variation, a plurality of tantalum layers 41 are not limited to have the same composition, but may have different compositions. a plurality of catalyst layers 42 are not limited to have the same composition, but may have different compositions.

In a variation, each a plurality of catalyst layers 42 of a composite layer 4 is not limited to be a porous layer. Alternatively, at least one catalyst layer 42, which is other than a catalyst layer 42 constituting a top layer of a plurality of catalyst layers 42, may be a porous layer, for example.

In a variation, each of a plurality of catalyst layers 42 of a composite layer 4 may be a non-porous layer.

In a variation, a plurality of recesses 5 may have the same shape. A method for producing such an electrode for electrolysis 1 may include an etching technique, a laser processing, or the like to form such a plurality of recesses 5. These techniques/processing can provide a greater degree of freedom for the design of a layout and dimensions of the plurality of recesses 5 and can realize a higher reproductivity about the positions of the plurality of recesses 5.

In a variation of an electrode for electrolysis 1 b shown in FIG. 8, the electrode for electrolysis 1 b has a plurality of recesses 5 recessed from a main surface 40, away from an intermediate layer 3, of a composite layer 4, and a top layer of the composite layer 4 is constituted by a tantalum layer 41. Components of the electrode for electrolysis 1 b similar to those of the electrode for electrolysis 1 are assigned same reference signs and explanation thereof may be omitted. According to the electrode for electrolysis 1 b including the composite layer 4 of which top layer is constituted by the tantalum layer 41, each of the plurality of recesses 5 has a depth such that each of the plurality of recesses 5 is made to go completely through at least one catalyst layer 42 of the plurality of catalyst layers 42. Each of the plurality of recesses 5 may preferably have a depth such that each of the plurality of recesses 5 is made to go completely through the plurality of catalyst layers 42, which can contribute to improve the chlorine generation efficiency.

In a variation, a bottom layer of a composite layer 4 may be constituted by one tantalum layer 41 of a plurality of tantalum layers 41 and be directly on a primary surface 21 of an electrically conductive substrate 2 without an intermediate layer 3 interposed therebetween. In this case, a top layer, furthest from the electrically conductive substrate 2, of the composite layer 4 may be constituted by one catalyst layer 42 of a plurality of catalyst layers 42.

(Recapitulation)

As can be seen from the foregoing description of the first and second embodiments and the like, the present disclosure discloses the following aspects.

An electrode for electrolysis (1; 1 a) according to a first aspect includes an electrically conductive substrate (2), an intermediate layer (3), and a composite layer (4). The electrically conductive substrate (2) contains at least titanium. The intermediate layer (3) is provided on a primary surface (21) of the electrically conductive substrate (2). The composite layer (4) is provided on the intermediate layer (3). The composite layer (4) includes a plurality of tantalum layers (41) and a plurality of catalyst layers (42). Each of the plurality of tantalum layers (41) is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. Each of the plurality of catalyst layers (42) contains platinum and iridium. The plurality of tantalum layers (41) and the plurality of catalyst layers (42) are alternately stacked one layer by one layer in a thickness direction (D1) of the electrically conductive substrate (2). A bottom layer of the composite layer (4) closest to the primary surface (21) of the electrically conductive substrate (2) is constituted by one tantalum layer (41) of the plurality of tantalum layers (41). A top layer of the composite layer (4) furthest from the electrically conductive substrate (2) is constituted by one catalyst layer (42) of the plurality of catalyst layers (42).

The electrode for electrolysis (1; 1 a) according to the first aspect is less likely to cause the separation of the composite layer (4).

The electrode for electrolysis (1; 1 a) according to a second aspect, which may be implemented in conjunction with the first aspect, the composite layer (4) has a main surface (40) away from the intermediate layer (3). The electrode for electrolysis (1; 1 a) has a plurality of recesses (5) recessed from the main surface (40) of the composite layer (4). Each of the plurality of recesses (5) has a depth which is greater than a distance (L1) between the main surface (40) of the composite layer (4) and a catalyst layer 42 (third catalyst layer 423), second furthest from the electrically conductive substrate (2), of the plurality of catalyst layers (42) and also is smaller than or equal to a distance (L2) between the main surface (40) of the composite layer (4) and the intermediate layer (3).

With the electrode for electrolysis (1; 1 a) according to the second aspect, a catalyst layer (third catalyst layer 423) second furthest from the electrically conductive substrate (2), of the plurality of catalyst layers (42), also can contribute to the generation of the chlorine. This allows the plurality of catalyst layers (42) to be gradually consumed from their sides constituting the plurality of recesses (5). The electrode for electrolysis (1; 1 a) according to this aspect thus can contribute to improve the durability and also can achieve the efficient consumption of the catalyst layer (42) to improve the chlorine generation efficiency by adjusting at least one of the number of catalyst layers (42) and the percentage of iridium contained in each of the plurality of catalyst layers (42).

The electrode for electrolysis (1 a) according to a third aspect, which may be implemented in conjunction with the second aspect, the depth of each of the plurality of recesses (5) is smaller than or equal to a distance (L3) between the main surface (40) and the bottom layer (first tantalum layer 411) of the composite layer (4).

The electrode for electrolysis (1 a) according to the third aspect is further less likely to cause the separation of the composite layer (4).

The electrode for electrolysis (1; 1 a) according to a fourth aspect, which may be implemented in conjunction with the second or third aspect, each of the plurality of recesses (5) is a crack extending linearly in a plan view as seen in the thickness direction (D1).

With the electrode for electrolysis (1; 1 a) according to the fourth aspect, a catalyst layer (42) farther from the electrically conductive substrate (2) tends to contribute to the generation of the chlorine and a catalyst layer (42) closer to the electrically conductive substrate (2) is less likely to be consumed. This can improve the durability of the electrode for electrolysis (1; 1 a).

The electrode for electrolysis (1; 1 a) according to a fifth aspect, which may be implemented in conjunction with the fourth aspect, each of the plurality of recesses (5) has a width (H1) within a range of 0.3 μm to 3 μm.

The electrode for electrolysis (1; 1 a) according to a sixth aspect, which may be implemented in conjunction with the fourth or fifth aspect, a percentage of S2 with respect to S1+S2 falls within a range of 5% to 50%, where S1 denotes an area of the main surface (40) of the composite layer (4) in the plan view as seen in the thickness direction (D1) of the electrically conductive substrate, and S2 denotes a total area of opening areas of the plurality of recesses (5) in the main surface (40) of the composite layer (4) in the plan view as seen in the thickness direction (D1) of the electrically conductive substrate.

The electrode for electrolysis (1; 1 a) according to a seventh aspect, which may be implemented in conjunction with the sixth aspect, at least one recess (5) of the plurality of recesses (5) is present in a 0.01 mm² square region in the plan view as seen in the thickness direction (D1) of the electrically conductive substrate (2), a total length of each opening edge of the at least one recess (5) present in the 0.01 mm² square region being greater than or equal to 1 mm.

The electrode for electrolysis (1; 1 a) according to an eighth aspect, which may be implemented in conjunction with any one of the first to seventh aspects, each of the plurality of catalyst layers (42) is a porous layer.

The electrode for electrolysis (1; 1 a) according to the eighth aspect can have an improved durability. The reason that the electrode for electrolysis (1; 1 a) according to the eighth aspect can have the improved durability may be inferred that each catalyst layer (42), other than a catalyst layer (42) that constitutes the top layer, of the plurality of catalyst layers (42) can easily contribute to generate the chlorine because the salt water is likely to infiltrate in an in-plane direction into the catalyst layer (42) through a side of this catalyst layer (42) exposed to the recess (5).

The electrode for electrolysis (1; 1 a) according to a ninth aspect, which may be implemented in conjunction with any one of the first to eighth aspects, the primary surface (21) of the electrically conductive substrate (2) is uneven.

The electrode for electrolysis (1; 1 a) according to the ninth aspect can improve the adhesion of the electrically conductive substrate (2) to the intermediate layer (3) and thus the composite layer (4) is less likely to be separated from the electrically conductive substrate (2).

A method for producing an electrode for electrolysis (1 a) according to a tenth aspect includes an intermediate layer formation process and a composite layer formation process. The intermediate layer formation process includes forming an intermediate layer (3) on a primary surface (21) of an electrically conductive substrate (2) containing titanium. The composite layer formation process includes forming a composite layer (4) on the intermediate layer (3). The composite layer (4) has a stacked structure alternating a plurality of tantalum layers (41) and a plurality of catalyst layers (42) one layer by one layer. Each of the plurality of tantalum layers (41) is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. Each of the plurality of catalyst layers (42) contains platinum and iridium. The composite layer formation process includes a first process, a second process, and a third process. The first process includes applying a solution containing tantalum onto the intermediate layer (3) and subsequently firing at a first prescribed temperature to form a tantalum layer (41), of the plurality of tantalum layers (41), that constitutes a bottom layer of the stacked structure. The second process includes repeating a first step and a second step to form a stacked body (401) serving as a basis of a remaining part, other than the tantalum layer (41) that constitutes the bottom layer, of the stacked structure. The first step includes applying a solution containing platinum and iridium and subsequently heating and drying at a second prescribed temperature to form a layer (second material layer 420) serving as a basis of one catalyst layer (42) of the plurality of catalyst layers (42). The second step includes applying a solution containing tantalum and subsequently heating and drying at a third prescribed temperature to form a layer (first material layer 410) serving as a basis of one, but other than the tantalum layer (41) that constitutes the bottom layer, of the plurality of tantalum layers (41). The third process includes firing the stacked body (401) at a fourth prescribed temperature, which is higher than each of the second prescribed temperature and the third prescribed temperature, to form the plurality of catalyst layers (42) and tantalum layers (41), other than the tantalum layer (41) that constitutes the bottom layer, of the plurality of tantalum layers (41) together with a plurality of cracks (recesses 5) recessed from a main surface (40) of the catalyst layer (42), the main surface (40) being a surface away from the intermediate layer (3).

The method for producing the electrode for electrolysis (1 a) according to the tenth aspect is less likely to cause the separation of the composite layer (4).

An electrode for electrolysis (1 b) according to an eleventh aspect includes an electrically conductive substrate (2), an intermediate layer (3), and a composite layer (4). The electrically conductive substrate (2) contains at least titanium. The intermediate layer (3) is provided on a primary surface (21) of the electrically conductive substrate (2). The composite layer (4) is provided on the intermediate layer (3). The composite layer (4) includes a plurality of tantalum layers (41) and a plurality of catalyst layers (42). Each of the plurality of catalyst layers (42) contains platinum and iridium. Each of the plurality of tantalum layers (41) is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. The plurality of tantalum layers (41) and the plurality of catalyst layers (42) are alternately stacked one layer by one layer in a thickness direction (D1) of the electrically conductive substrate (2). A bottom layer of the composite layer (4) closest to the primary surface (21) of the electrically conductive substrate (2) is constituted by one tantalum layer (41) of the plurality of tantalum layers (41). A top layer of the composite layer (4) furthest from the electrically conductive substrate (2) is constituted by another one tantalum layer (41) of the plurality of tantalum layers (41). The composite layer (4) has a main surface away from the intermediate layer (3). The electrode for electrolysis (1; 1 a) has a plurality of recesses (5) recessed from the main surface (40) of the composite layer (4). Each of the plurality of recesses (5) has a depth such that each of the plurality of recesses (5) is made to go completely through at least one catalyst layer (42) of the plurality of catalyst layers (42).

The electrode for electrolysis (1 b) according to the eleventh aspect is less likely to cause the separation of the composite layer (4).

An electrode for electrolysis (1; 1 a) according to a twelfth aspect includes an electrically conductive substrate (2), an intermediate layer (3), and a composite layer (4). The electrically conductive substrate (2) contains at least titanium. The intermediate layer (3) is provided on a primary surface (21) of the electrically conductive substrate (2). The composite layer (4) is provided on the intermediate layer (3). The composite layer (4) includes a plurality of tantalum layers (41) and a plurality of catalyst layers (42). Each of the plurality of tantalum layers (41) is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum. Each of the plurality of catalyst layers (42) contains platinum and iridium. The plurality of tantalum layers (41) and the plurality of catalyst layers (42) are alternately stacked one layer by one layer in a thickness direction (D1) of the electrically conductive substrate (2). A bottom layer of the composite layer (4) closest to the primary surface (21) of the electrically conductive substrate (2) is constituted by a tantalum layer (41). Atop layer of the composite layer (4) furthest from the electrically conductive substrate (2) is constituted by a catalyst layer (42).

The electrode for electrolysis (1; 1 a) according to the twelfth aspect is less likely to cause the separation of the composite layer (4).

REFERENCE SIGNS LIST

-   -   1, 1 a, 1 b electrode for electrolysis     -   2 electrically conductive substrate     -   21 primary surface     -   3 intermediate layer     -   4 composite layer     -   40 main surface     -   41 tantalum layer     -   42 catalyst layer     -   5 recess     -   401 stacked body     -   L1 distance     -   L2 distance     -   L3 distance 

1. An electrode for electrolysis, comprising: an electrically conductive substrate containing at least titanium; an intermediate layer on a primary surface of the electrically conductive substrate; and a composite layer on the intermediate layer, the composite layer including a plurality of tantalum layers each of which is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum, and a plurality of catalyst layers each of which contains platinum and iridium, the plurality of tantalum layers and the plurality of catalyst layers being alternately stacked one layer by one layer in a thickness direction of the electrically conductive substrate, a bottom layer of the composite layer closest to the primary surface of the electrically conductive substrate being constituted by one tantalum layer of the plurality of tantalum layers, and a top layer of the composite layer furthest from the electrically conductive substrate being constituted by one catalyst layer of the plurality of catalyst layers.
 2. The electrode for electrolysis of claim 1, wherein the composite layer has a main surface away from the intermediate layer, the electrode for electrolysis has a plurality of recesses recessed from the main surface of the composite layer, each of the plurality of recesses has a depth which is: greater than a distance between the main surface of the composite layer and a catalyst layer, second furthest from the electrically conductive substrate, of the plurality of catalyst layers; and also smaller than or equal to a distance between the main surface of the composite layer and the intermediate layer.
 3. The electrode for electrolysis of claim 2, wherein the depth of each of the plurality of recesses is smaller than or equal to a distance between the main surface and the bottom layer of the composite layer.
 4. The electrode for electrolysis of claim 2, wherein each of the plurality of recesses is a crack extending linearly in a plan view as seen in the thickness direction.
 5. The electrode for electrolysis of claim 4, wherein each of the plurality of recesses has a width within a range of 0.3 μm to 3 μm.
 6. The electrode for electrolysis of claim 4, wherein a percentage of S2 with respect to S1+S2 falls within a range of 5% to 50%, where S1 denotes an area of the main surface of the composite layer in the plan view as seen in the thickness direction of the electrically conductive substrate, and S2 denotes a total area of opening areas of the plurality of recesses in the main surface of the composite layer in the plan view as seen in the thickness direction of the electrically conductive substrate.
 7. The electrode for electrolysis of claim 6, wherein at least one recess of the plurality of recesses is present in a 0.01 mm² square region in the plan view as seen in the thickness direction of the electrically conductive substrate, a total length of each opening edge of the at least one recess present in the 0.01 mm² square region being greater than or equal to 1 mm.
 8. The electrode for electrolysis of claim 1, wherein each of the plurality of catalyst layers is a porous layer.
 9. The electrode for electrolysis of claim 1, wherein the primary surface of the electrically conductive substrate is uneven.
 10. A method for producing an electrode for electrolysis, comprising: an intermediate layer formation process including forming an intermediate layer on a primary surface of an electrically conductive substrate containing titanium; and a composite layer formation process including forming a composite layer on the intermediate layer, the composite layer having a stacked structure alternating a plurality of tantalum layers and a plurality of catalyst layers one layer by one layer, each of the plurality of tantalum layers being made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum, each of the plurality of catalyst layers containing platinum and iridium, the composite layer formation process including a first process, a second process, and a third process, the first process including applying a solution containing tantalum onto the intermediate layer and subsequently firing at a first prescribed temperature to form a tantalum layer, of the plurality of tantalum layers, that constitutes a bottom layer of the stacked structure, the second process including repeating a first step and a second step to form a stacked body serving as a basis of a remaining part, other than the tantalum layer that constitutes the bottom layer, of the stacked structure, the first step including applying a solution containing platinum and iridium and subsequently heating and drying at a second prescribed temperature to form a layer serving as a basis of one catalyst layer of the plurality of catalyst layers, the second step including applying a solution containing tantalum and subsequently heating and drying at a third prescribed temperature to form a layer serving as a basis of one, but other than the tantalum layer that constitutes the bottom layer, of the plurality of tantalum layers, the third process including firing the stacked body at a fourth prescribed temperature, which is higher than each of the second prescribed temperature and the third prescribed temperature, to form the plurality of catalyst layers and tantalum layers, other than the tantalum layer that constitutes the bottom layer, of the plurality of tantalum layers together with a plurality of cracks recessed from a main surface of the catalyst layer, the main surface being a surface away from the intermediate layer.
 11. An electrode for electrolysis, comprising: an electrically conductive substrate containing at least titanium; an intermediate layer on a primary surface of the electrically conductive substrate; and a composite layer on the intermediate layer, the composite layer including a plurality of tantalum layers each of which is made from tantalum oxide, tantalum, or a mixture of tantalum oxide and tantalum, and a plurality of catalyst layers each of which contains platinum and iridium, the plurality of tantalum layers and the plurality of catalyst layers being alternately stacked one layer by one layer in a thickness direction of the electrically conductive substrate, a bottom layer of the composite layer closest to the primary surface of the electrically conductive substrate being constituted by one tantalum layer of the plurality of tantalum layers, a top layer of the composite layer furthest from the electrically conductive substrate being constituted by another one tantalum layer of the plurality of tantalum layers, the composite layer having a main surface away from the intermediate layer, the electrode for electrolysis having a plurality of recesses recessed from the main surface of the composite layer, each of the plurality of recesses having a depth such that each of the plurality of recesses is made to go completely through at least one catalyst layer of the plurality of catalyst layers. 